1. Field of the Invention
The present invention relates to an exhaust gas purification system of an internal combustion engine having a particulate filter. Specifically, the present invention relates to an exhaust gas purification system of an internal combustion engine capable of performing regeneration of a particulate filter at proper timing.
2. Description of Related Art
In recent years, reduction of exhaust emission of internal combustion engines installed in automobiles and the like has been required. Specifically, in a compression ignition diesel engine whose fuel is light oil, it is necessitated to eliminate not only gas such as carbon monoxide, hydrocarbon and nitrogen oxides, but also exhaust particulate matters such as soot and soluble organic fractions contained in exhaust gas. Therefore, a particulate filter is disposed in an exhaust passage for collecting the exhaust particulate matters in the exhaust gas.
If the exhaust gas flows into the particulate filter, the exhaust gas passes through porous partition walls of the particulate filter. At that time, the exhaust particulate matters contained in the exhaust gas are collected by surfaces and pores of the partition walls. If a quantity of the collected and deposited particulate matters increases excessively, a flow resistance in the particulate filter will increase and a back pressure of the engine will increase. As a result, an output of the engine is reduced. Therefore, the exhaust particulate matters deposited on the particulate filter should be occasionally eliminated from the particulate filter to regenerate the particulate filter and to recover the ability of the particulate filter to make the exhaust gas flow downstream.
A certain system, which can perform the regeneration of the particulate filter while the engine is operating, has an oxidation catalyst such as platinum disposed on the particulate filter. The system performs a post-injection for supplying the fuel into the particulate filter in an exhaustion stroke. Thus, the system oxidizes and eliminates the deposited exhaust particulate matters, which are more difficult to oxidize than the injected fuel, by utilizing combustion heat of the fuel.
If the regeneration of the particulate filter is performed frequently, a fuel cost will increase. If a time interval to the next regeneration is too long, the quantity of the deposited exhaust particulate matters will increase excessively, and the deposited exhaust particulate matters will be suddenly burned in the regeneration process. In such a case, there is a possibility that the particulate filter is heated to abnormally high temperature and is damaged. Therefore, the regeneration timing of the particulate filter should be preferably decided by determining the deposited state of the exhaust particulate matters. A system disclosed in Unexamined Japanese Patent Application Publication No. 2003-27919 or No. 2003-83035 senses a differential pressure between an inlet and an outlet of the particulate filter and determines that the regeneration timing is reached when the sensed value of the differential pressure exceeds a predetermined value. This scheme is based on a fact that the differential pressure increases in accordance with the increase in the flow resistance due to the increase of the quantity of the deposited exhaust particulate matters on the particulate filter.
The technology for determining the deposited state of the exhaust particulate matters based on the differential pressure utilizes the fact that the flowing state of the exhaust gas inside the particulate filter changes in accordance with the deposited state of the exhaust particulate matters. Therefore, even if the deposited state is the same, the sensed deposited state will vary if the flowing state of the exhaust gas in the particulate filter varies. In such a case, there is a possibility that the regeneration timing of the particulate filter delays or the regeneration frequency increases.